Optimal intervals differ for double knock application of paraquat after glyphosate or haloxyfop for improved control of Echinochloa colona, Chloris virgata and Chloris truncata

The grasses Echinochloa colona (L.) Link, Chloris virgata Sw. and C. truncata R. Br. are major problems in summer fallows of the sub-tropical grain region of Australia. Traditionally, these weeds were treated with glyphosate alone, but E. colona and C. truncata populations have evolved glyphosate resistance, and the weed flora is also being dominated by the glyphosate-tolerant species C. virgata. For improved control of these populations, sequential application of glyphosate, or recently haloxyfop, followed by paraquat is being used. The optimal interval between sequential applications of these herbicides needs to be defined for these summer-growing grasses. Pot experiments were conducted using glyphosate or haloxyfop followed by paraquat at intervals from 1 to 21 days. In addition, populations of E. colona and C. truncata with resistance to glyphosate were compared. The optimal interval between sequential applications differed for the three grasses and the herbicide used for the first application. For the glyphosate-paraquat sequential treatments, the optimal intervals were 1-14 days for GS and GR E. colona, 7 days for C. virgata, and 7-14 days and 14 days for GS and GR C. truncata populations. For the haloxyfop-paraquat treatments, the optimal intervals were 1-21 days for E. colona, 1-4 days for C. virgata and 1-7 days for C. truncata. This treatment achieved 100% control irrespective of resistance status. Thus, sequential application, particularly haloxyfop followed by paraquat is a highly effective tactic for control of these three weeds.